Examining the physico-chemical, structural and thermo-mechanical properties of naturally occurring Acacia pennata fibres treated with KMnO4

Natural fiber is a viable and possible option when looking for a material with high specific strength and high specific modulus that is lightweight, affordable, biodegradable, recyclable, and eco-friendly to reinforce polymer composites. There are many methods in which natural fibres can be incorporated into composite materials. The purpose of this research was to evaluate the physico-chemical, structural, thermal, and mechanical properties of Acacia pennata fibres (APFs). Scanning electron microscopy was used to determine the AP fibers' diameter and surface shape. The crystallinity index (64.47%) was discovered by XRD. The irregular arrangement and rough surface are seen in SEM photos. The findings demonstrated that fiber has high levels of cellulose (55.4%), hemicellulose (13.3%), and low levels of lignin (17.75%), which were determined through chemical analysis and validated by Fourier Transform Infrared Spectroscopy (FTIR). By using FTIR, the functional groups of the isolated AP fibers were examined, and TG analysis was used to look into the thermal degrading behaviour of the fibers treated with potassium permanganate (KMnO4) Due to their low density (520 kg/m3) and high cellulose content (55.4%), they have excellent bonding qualities. Additionally, tensile tests were used for mechanical characterisation to assess their tensile strength (685 MPa) and elongation.


Materials
The Acacia pennata fiber was treated using destilled water, NaOH pellets, and a potassium permanganate pellets in acetone.This was acquired from Premier Chemicals in Nagercoil, Kanniyakumari district, Tamil Nadu, India.

Extraction method of Acacia pennata fiber from the plant
Acacia pennata is a large scrambling or climbing shrub.It can grow up to a height of almost 100 m.Its trunk and branches are prickly and smooth.The stem of Acacia pennata fibers were gathered from Tamilnadu-Kerala border (near Panachamoodu and Vellarada area).The fiber has been separated from its bark of the plant and then allowed to dry at ambient temperature (27 °C) for few days.The dried fibers are treated with distilled water for about 20 min for microbial degradation before treating the fiber with alkali (NaOH).The dried fiber sections were pre-treated with NaOH aqueous solution and the potassium permanganate (KMnO 4 ) solution was prepared with the help of acetone and KMnO 4 pellets.The importance of this treatment for improving surface properties was also addressed in depth.This allowed us to reduce the hydrophilic capacity and increase the adhesion between the fibers and polymers 36 .The fibers were soaked in this (0.1 M NaOH) solution for 20 min.The fibers were removed after 20 min and dried for 10 to 15 days at room temperature (27 °C).After drying, the fiber sections are immersed in KMnO 4 solution for about 15 minitues 37 .Then, these fibers were allowed to dry at ambient temperature.The dried AP fibers were chopped into powder form or broken into little fiber strips based on the need of analysis.KMnO 4 treated AP fibers were packed in zip-lock cover and store in room temperature.Figure 1 shows the pictures of Acacia pennata plant fiber and its chemical treatments.

Experimental techniques
Powder XRD technique Powder X-ray diffraction (PXRD) is a method for determining if a material is crystalline or amorphous 38 .It is a quick analytical method that can reveal the size of unit cells and is mostly used to determine a crystalline material's phase.Copper (Cu) is the most common target material for powder x ray diffraction, and the powder sample were subjected to X-ray diffraction using CuKα (i.e., CuKα is the emission of copper) radiation with a wavelength of 1.5406 nm in a Bruker x-ray diffractometer.A 2θ range X-ray diffraction examination was performed with angles ranging from 3° to 70°.In the spectrum of the APFs, the integrated intensities of the Bragg peaks were recognised, and their crystallinity indices were calculated.The crystallinity index (CI) of the natural fiber was measured using the traditional peak height technique developed by Segal et al 39 .
where, H 002 is the height of the crystalline peak situated around 22° and 23°; H am is the height of amorphous peak situated around 14° and 16°.
The crystallite size (CS) of the natural fiber were calculated through the following equation 40 ; where, β is the full-width at half-maximum (FWHM) value of peaks; θ is the Bragg's angle 41 .

FTIR techniques
A crucial method for locating significant groups is Fourier-transform infrared (FTIR) spectroscopy 42 ."Perkin Elmer Spectrum Two" FTIR spectrometer with a scan rate of 32 per minute, a resolution of 2 cm −1 , and a wave number range between 4000 cm −1 and 400 cm −1 were used to obtain the FTIR spectra of the APFs.

SEM techniques
A scanning electron microscope (SEM) with EDS is used to investigate the topography and morphology of the surfaces of materials as well as biological samples.EDS is used to facilitate elemental recognition.A concentrated electron beam is used to scan a sample's surface in a scanning electron microscope (SEM), which creates images of the sample.The spatial correlations between the various matrices' and reinforcement fibers' constituent parts have been clarified by SEM studies 43 .Using a scanning electron microscope, the surface morphology of APFs was studied (JEOL JSM-6390LV) [44][45][46] .Magnification of the JEOL JSM-6390LV is on the order of 3,00,000 × with high resolution 3.0-nm, where fine details of the specimens can be observed.

Thermo-gravimetric techniques
Thermo-gravimetric analysis by Perkin Elmer was used to access the thermal stability behaviour of APFs.The curve plots the temperature difference between the reference material and the sample material against time or temperature.The amount and rate of change in a material's weight as a function of temperature or time in an environment of nitrogen, helium, air, another gas, or in a vacuum are measured using thermo-gravimetric analysis (TG).The method can identify materials that experience weight gains or loss as a result of oxidation, dehydration, or other processes 47 .

Density using pycnometer
The thickness (density) of natural fibers is frequently measured with a pycnometer 48 .The fiber sample is simply dried at room temperature before use to remove moisture 49 .If moisture remains in the fiber material, a vacuum desiccator can be employed to eliminate it completely.The samples are then thoroughly pulverised and placed in the pycnometer to measure the density 50 .Toluene is used as an immersion solvent when measuring the densities of untreated and KMnO 4 treated AP fibres in accordance with ASTM D578-89 standard.The fibres must be soaked in toluene for at least two hours before being weighed to assess their density.Density of APF is derived from; (1) where, m 1 -mass of dry empty pycnometer (g); m 2 -mass of pycnometer + fiber (g); m 3 -mass of pycnometer + toluene (g); m 4 -mass of pycnometer + toluene + fiber (g); ρt-density of toluene (0.867 g/cm 3 ); ρf-density of natural fiber in g/cm  .

CHNS (elemental) analysis techniques
Using a CHNS analyzer, model Elementar Vario EL III Instruments based on the principle of Dumas's method 58 , which involves the complete and quick oxidation of the sample by "flash combustion," one may determine the percentages of C, H, N, S, (carbon, hydrogen, nitrogen, sulphur and oxygen elements) in organic compounds.
To provide carbonate and organic carbon and to get a general idea of the composition of the organic matter (i.e., to distinguish between marine and terrigenous sources, based on total organic carbon / total nitrogen [C/N] ratios), elemental analyses of total nitrogen and carbon (and sulphur) are conducted.

Tensile strength analysis
There are various reasons to undertake tensile tests.Typically, Zwick/Roell 59 specimens are used for the tensile test 60 .The maximum stress that the material can withstand or the stress required to generate substantial plastic deformation are two ways to assess the strength of an object of interest.A computerised tensile testing machine was used for the tensile testing analysis.The tensile mechanical characteristics of a material are described in depth via tensile testing and also these tests were performed with ASTM-D412 international standards 59 .These characteristics can be represented graphically as a stress/strain curve to display information such as the point at which the material failed and to provide information on characteristics such the elastic modulus, strain, and yield strength 61 .The tests are conducted at a temperature of 21 °C, a cross-head speed of 30 mm/min, and a relative humidity of 65 ± 2% 62 .To confirm the accuracy of the tensile test, 10 fibers, each 50 mm long, are tested [63][64][65][66][67][68][69][70][71] .Records are kept of the average tensile strength, elongation at break, and strain rate.The following empirical relationship governs the tensile strength of the AP fibers; where; F is the force in Newtons, A is cross-sectional area in mm 2 and T is the tensile strength in MPa 72,73 .
Stress-strain curves are used to evaluate the mechanical characteristics of AP fibers, including their tensile strength and percentage of elongation 70,74 .A microscope is used to conduct a comprehensive longitudinal direction of the chosen acacia pennata fibers in order to measure their average diameter.Additionally, from the SEM images, the thickness of the fibers is calculated using the ImageJ software 59 .Nearly the same diameter was obtained by both techniques.The shape of APFs cross section is round 53,[75][76][77] .The angle formed by the helical winding orientations of cellulose microfibrils is known as the microfibril angle (MFA).A plant fibers strength and stiffness are often affected by the amount of cellulose and the spiral-shaped wink.The Global deformation equation is used to determine the microfibrillar angle (α) of AP fibers.
The term "Youngs modulus" is used to describe a material's resistance to elastic deformation under load.It displays the strength of a material, to put it another way.
where, F-applied force; E-young's modulus (GPa) of the fiber; A-cross-sectional area of the fiber and ∆L/ Lo-ratio of elongation 74 .
The elongation at break is the ratio of the modified length to the original length when a test specimen is fractured.It demonstrates how naturally occurring plant fiber may withstand changes without breaking.It is possible to calculate the elongation at break of the AP fiber in accordance with ISO/IEC 17,025 80,81 tensile test.

Chemical analysis test
Chemical analysis tests were used to identify the chemical composition of natural fibers and how it affected their mechanical qualities.To study the percentages of chemical compositions (cellulose, hemicelluloses, lignin, pectin, wax, and ash) which present in the natural fiber using chemical analysis.These are the primary components of natural fibers.All chemical analysis tests were performed in accordance with ASTM-D3822 and IS199 international standards 82 .Both natural and synthetic fibers are currently used in the production of engineering materials.As a result, their mechanical and thermal characteristics depend on the environment.Therefore, indepth research must be done to analyse the characteristics of natural fibers.

Specimen collection
The Acacia pennata plant has spread and grown throughout Kerala and Tamil Nadu in India.Acacia pennata plant for the research purposes are collected from the authors farm at Panchamoodu and Vellarada. (

Guidelines and regulations
All testing was performed in accordance with the relevant ISO standards.The methods and procedures used were compliant with the guidelines and regulations outlined in the ISO standard to ensure accurate and reliable results.

Powder XRD analysis
Figure 2 depicts the X-ray diffraction (XRD) pattern of the untreated and KMnO 4 -treated AP fibers.It (untreated APF) shows the crystalline peak (22.59°) on the crystallographic plane (002) and amorphous peak (15.25°) on the lattice plane (110), which demonstrates the semi-crystalline nature 77 .This is because hemicellulose, lignin, and pectin are present.Two well-defined diffraction peaks are observed in KMnO 4 treated APFs around 2θ = 22.73° and the amorphous peak at 2θ = 15.37°include lignin, pectin, hemicellulose, and amorphous cellulose, which contains a larger percentage of amorphous fraction 83 .The crystallinity index of KMnO 4 treated fiber was determined as 64.47%.The high crystallinity index of AP fiber was caused by the efficient removal of contaminants and hemicellulose.It was slightly increased when compared to untreated (46.52%)AP fiber.The CI of permanganate treated AP fibers are significantly greater than Kapok (45%) fiber and substantially lower than crushed guadua (65.3) fiber, jute (71%) and hemp (88%) fiber 31,84 .However, using the well-known scherrer formula, the crystallite size (CS) of the KMnO 4 treated APFs was found to be 6.75 nm and it was higher than that of untreated (1.9 nm) AP fiber.The crystalline size value is lower than that of ramie fibres (16 nm) and higher than that of cotton fibers (5.5 nm), tamarindus indica fruit fibers (5.73 nm), ferula communis (1.6 nm), and carbon fibers (0.669 nm) 85,86 .Table 1 represents the comparison of CI and CS values of raw and KMnO 4 treated APFs with other natural fibers.

SEM analysis
A test method called scanning electron microscopy uses an electron beam to magnify and examine a material.The surface shape of the untreated and KMnO 4 treated AP fiber is shown in Fig. 4a,b.It was observed that the APF treated with KMnO 4 had a rough and disorganised surface shape compared with untreated APFs.It reveals that the hemicellulose content (white layer on the surface of untreated SEM image), void spaces, holes, parenchyma cells and few impurities are visible on its surface.To enhance the interfacial adhesion with polymer matrices, these hemicelluloses and lignin, wax like impurities, amorphous content should be removed, and it was done by KMnO 4 treatment.Structural properties and its modifications done by this treatment reveals that the experimental fibers can act as a good reinforcement material in composite manufacturing.

Thermo-gravimetric analysis
The weight loss of composites in relation to temperature increase was quantified using the thermo-gravimetric analysis.Greater thermal stability results from higher decomposition temperatures 104 .The TG and DTG curves of the KMnO 4 treated APF sample was shown in Figs. 5 and 6.According to the graph, the degradation peaks are related to moisture evaporation, the breakdown of cellulose, lignin, wax, and other contaminants, as well as hemicellulose.At a temperature of about 40 °C to 120 °C, nearly 12% of weight loss has been occurred, which was mainly depending upon the moisture content in the untreated APF sample.Then the second major degradation occurs between 120 °C and 280 °C temperature range with the reduction of 18.5%.This weight loss has been observed due to the degradation of hemicellulose component.Within the temperature range between 280 °C and 400 °C, 25.46% of weight has been reduced.The destruction of the cellulose content's glycosidic linkages was the cause of the significant weight loss.Another drop was observed at the temperature within the range from 400 °C to 500 °C, nearly 20.93% of loss has occurred, which may be due to the degradation of lignin contents..07 618.17Out of plane of -OH bonding 58,82 In the DTG (Derivative thermo-gravimetric) curve, where the weight loss abruptly occurs at 90.7 °C with a weight loss of 0.283 mg/min, clearly shows the removal of moisture content within the fiber and the second and third degradation peaks were observed at the temperatures of 259.4 °C, 302.6 °C, 441.2 °C with the weight losses of 0.478 mg/min and 0.529 mg/min, 0.462 mg/min.Compared to hemicellulose and cellulose degradation, the thermal decomposition of lignin occurs over a wider temperature range, starts earlier, and goes up to higher temperatures 400 °C105 .An abrupt drop in the DTG curve, which is related to the thermal breakdown of hemicelluloses and glycosidic linkages in cellulose, serves as an indicator of this 106 .Hemicellulose is a type of polysaccharide that is linked to cellulose and contains various sugar units.Compared to cellulose, it has a higher degree of chain branching but significantly lower levels of polymerization.Hemicellulose thermal degradation occurs before that of cellulose, although its impact is proportionally reduced by the amount of hemicellulose in the fiber 107 .
The next deterioration peak occurred at 603.9 °C with weight losses of 0.040 mg/min, which may have been caused by the breakdown of the fiber's lignin and wax components.Lignin is a complex hydrocarbon polymer that contains both aliphatic and aromatic components 108 .Compared to the thermal decomposition of hemicellulose and cellulose, the thermal decomposition of lignin occurs over a wider range, starts earlier, and goes to higher  www.nature.com/scientificreports/temperatures.The lesser amount of fiber present, however, also limits its impact 107 .When the temperature ranges from ambient temperature to more than 600 °C, the complex aromatic ring component of the lignin structure decomposes with a minimum weight loss rate 9, 109 .Tables 3 and 4 depicts the comparison between the thermal study (3) and Mass loss at Tmax (4) of untreated and KMnO 4 treated AP fibers.

Physical analysis
The density of the fiber treated with untreated and KMnO4 was estimated to be 1090 kg/m 3 and 520 kg/m 3 (Table 6).Cavities and holes were removed during alkalization 110 .So that, density of the optimally treated APF were slightly decreased.However, the density is slightly lower than that of the Acacia leucophloea 1385 kg/m 3 , coir fiber 1200 kg/m 322 .Due of the uneven profiles of bark fibers, diameter determination in the stem of AP fiber is rather difficult.The measured diameter of the ACF was 299.39 μm which was confirmed from SEM images.The Comparison of diameter and density values of untreated and KMnO 4 treated APFs with other natural fibers are represented in Table 5.

CHNS analysis
The Dumas's method 58 , which entails the total and rapid oxidation of the sample by "flash combustion," is the basis for the CHNS analyser, which is used to determine the percentages of carbon, hydrogen, nitrogen, and sulphur in organic compounds.The Dumas method is a technique for calculating the quantity of chemical compounds (elements).This approach is most useful for figuring out how much C, H, N, and S are present in organic compounds, which typically ignite at 1800 °C.The weight percentage of each chemical in the KMnO 4 treated sample is displayed in Table 6.
According to CHNS study, untreated AP fibres have a carbon content of 43.38%, however after being treated with permanganate, the carbon content decreases to 33.22%.One of the most vital aspects to alter the mechanical and tribological qualities of the final product is high carbon content in the natural fibers.Both samples can be employed as conductive fillers in dielectric loss materials because of the carbon content is above 30% in both.The average composition of carbon and hydrogen in chicken feather fibres are 47.4% and 7.2% 116 .It is found that the carbon content of coconut shell fibres and sugarcane bagasse fibres are 46.7% and 44.7%.These values are comparable to AP fibers.

Chemical analysis
Table 7 provides the comparison table of chemical composition of untreated, KMnO 4 treated with different existing fibers.After pre-treatment, the cellulose content of the plant fibers generally increased.Due to the crystalline areas' altered lattice structures, the APF has a cellulose content of 55.4% 117 .These fibers have lower cellulose levels than Acacia Concinna fiber (59.43 wt%), Acacia leucophloea (68.09 wt% to 76.69 wt%), and Prosopis Juliflora fiber (61.65 wt% to 72.27 wt%) 93 and larger than that of coir fiber (32-43%) and Ficus leaf fiber (38.1%) 93,118 Hemi-Cellulose of the APF was decreased (13.30%) in this treatment.This hemicellulose content was much larger than that of Acacia concinna fiber (12.78%) and Acacia planifrons (9.41%) etc.The diffusion of lignin in KMnO 4 solution was blamed for the significant change in the lignin concentration (17.75%).However, it is soluble in hot alkali, rapidly oxidised, and condensable with phenol.Lignin is not hydrolyzed by acids 119 .After this treatment, pectin levels similarly dropped (1.9%).Wax content (0.79%) of the KMnO4-treated APF dropped as well, which is a favourable change.As opposed to plant fiber with higher wax content, fiber from plants with   120 .The amount of moisture (13.4%) in the KMnO4-treated APF decreased as well.The ash content of the APF, on the other hand, was subtly increased from 10%, which supported the growth of the crystalline component in the fiber 110 .

Tensile strength
One of the most often investigated features of natural fiber reinforced composites is tensile strength.When choosing a particular natural fiber for a given application, the fiber strength can be a crucial consideration 126 .Tensile testing of individual technical fibers is a standard method for determining the tensile characteristics of natural fibers 127 .The test was conducted at an ambient temperature of 21 °C, a relative humidity of approximately 65%, and a specimen gauge length of 50 mm 100 .The comparison of tensile strength, young's modulus, micro-fibrillar angle, and breaking elongation of the untreated and KMnO 4 -treated APFs with other natural fibers are shown in Table 8.According to the computed data, the tensile strength of the untreated and KMnO 4 treated APFs were found to 181.69 MPa and 685 MPa with 6.2%, 4.1% elongation and young's modulus 29.3GPa, 16.707 GPa respectively.The tensile strength of the jute fiber is (400-800 MPa) and its young's modulus is (10-30 GPa) 128 .As a result, the AP fiber's tensile strength and young's modulus were almost on par with those of jute fiber.The cell walls structure and chemical makeup of bark fibers, particularly the amount of cellulose, have a significant impact on their mechanical properties 129 .

Conclusion
The paper discusses the outcomes of analyses performed on the KMnO 4 -treated AP fiber using X-ray diffraction (XRD), Fourier Transform Infrared (FTIR), scanning electron microscopy (SEM), thermo-gravimetric analysis (TGA), and mechanical (tensile strength) analysis.
(1) The mechanical analysis of AP fiber makes it a dependable and long-lasting material for building composite fiber materials and fiber reinforced concrete for use in construction.Due to the orientation of the fibers, AP fibers have superior mechanical properties (such strength and young's modulus), which improves their capacity to handle loads and stress.(2) The KMnO 4 -treated AP fiber's X-ray diffraction (XRD) patterns revealed a semi-crystalline structure, with a crystallinity index of 51.63%.These findings imply that the KMnO 4 treatment was successful in eliminating impurities and raising the AP fiber's crystallinity index.The crystallite size of the KMnO 4 -treated AP fiber was 1.05 nm.Overall, the results of this study provide valuable information on the effects of KMnO 4 treatment on the structure, stability and mechanical analysis of AP fiber.(3) FTIR spectral analysis revealed the existence of lignin, saline content, and components of cellulose and hemicellulose.Scanning electron microscopy revealed that the KMnO 4 -treated AP fiber's surface was rough and disordered, with amorphous content and impurities clearly apparent.Chemical analysis outcomes showed that APF has the higher cellulose (55.4%) and lesser hemicellulose (13.3%) content.(4) The degradation of moisture, cellulose, lignin, wax, and other impurities, as well as hemicellulose, corresponded to the peaks in the thermo-gravimetric study.This thermo-gravimetric analysis showed that the KMnO 4 treated AP fiber has a higher thermal stability (337.47 °C), with higher decomposition temperatures (223.5 °C), making it a potential candidate for use in composite materials.(5) This fiber's overall structure and stability have improved as a result of the KMnO 4 treatment.Therefore, with further purification, AP fibre has the potential to be used as a reinforcement material in the creation of composites.All the above findings and lower density (520 kg/m 3 ) of the APF would make them suitable for lightweight composite materials.
The experiments conducted on KMnO4 treated Acacia pennata fibers show remarkable mechanical properties, such as tensile strength of 685 MPa and a Young's modulus of 16.707 GPa.The high tensile strength of Acacia pennata fibers has a possibility of utilizing as particle replacement of cementitious material in the construction as well as interior structural components.In the current practices many natural fibers are utilized in the construction sector.Furthermore, these composites have high potential to be applied in the sandwish structure due to its light weight with higher flexural properties.The fibers microfibrillar angle of 16.134º and break elongation of 4.1% further support their suitability for these applications.However, it is crucial to evaluate these results against industry standards to fully assess the potential of the Acacia pennata fibers applications.Further research should be conducted to examine their feasibility and performance against existing materials in the construction and composite industries.

Figure 3 Figure 2 .
Figure3displays the FTIR spectra of the AP fiber that had been treated with KMnO 4 .From graph, the hydrogenbonded OH stretching group of the water molecules is responsible for the prominent peak at 3435 cm −1 .The CH stretching vibrations in the cellulose and hemicellulose components are visible in the peak at 2924.41 cm −1 .The sharp and medium peak, which is associated with the C=H stretching, lies at a height of 1645.35 cm −193 .A hydrogen bond may be seen at the peak at 1420.62 cm −1 .The C = O stretching of lignin is what causes the absorption band at 1030.61 cm −1 to exist.The presence of saline content may be seen in the lower peak at 778.48 cm −1 .The peak at 629.07 cm −1 demonstrates the region of -OH bending was found at out of plane, proving that the chemical

Figure 5 .
Figure 5. TG curves of untreated and KMnO 4 treated AP fiber.

Table 1 .
Comparison table for CI and CS values of raw and KMnO 4 treated APFs with other natural fibers.'s findings are supported by the elimination of lignin and hemicelluloses from the KMnO 4 treated APFs.The FTIR vibrational band assignments of untreated and KMnO 4 treated AP fibers were shown in Table2.

Table 2 .
FTIR Vibrational band assignments of untreated KMnO 4 treated AP fibers.

Table 3 .
Thermal study of untreated and KMnO 4 treated AP fibers.

Table 4 .
Mass loss at Tmax of untreated and KMnO 4 treated AP fibres.

Table 5 .
Comparison table for diameter and density values of raw and KMnO 4 treated APFs with other natural fibers.

Table 6 .
Weight percentage of C, H, N, S in untreated and KMnO 4 treated AP fiber.ND not detected.

Table 7 .
Comparison table of chemical compositions of APFs with other existing fibers.

Table 8 .
Comparison table for Tensile strength of untreated and KMnO 4 treated APFs with other natural fibers.